1. Field of the Invention
Implementations of various technologies described herein generally relate to seismic data acquisition, particularly time-lapse seismic data acquisition.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
In a typical seismic survey, a plurality of seismic sources, such as explosives, vibrators, airguns or the like, may be sequentially activated at or near the surface of the earth to generate energy which may propagate into and through the earth. The seismic waves may be reflected back by geological formations within the earth. The resultant seismic wavefield may be sampled by a plurality of seismic sensors, such as geophones, hydrophones and the like. Each sensor may be configured to acquire seismic data, normally in the form of a record or trace representing the value of some characteristic of the seismic wavefield against time. The acquired seismic data may be transmitted over electrical or optical cables to a recorder system. The recorder system may then store, analyze, and/or transmit the data. This data may be used to detect the possible presence of hydrocarbons, changes in the subsurface, and the like.
In a typical time-lapse seismic survey, a second or monitor survey may be performed in the same location as a previous baseline survey for the purpose of comparing the images produced by the two surveys. Typically, the sources may be activated at the same locations and the sensors may be located at the same locations in both surveys. The images may be subtracted to create the time-lapse difference image. A time-lapse difference image represents any change to the subsurface layers since the baseline survey was performed. For example, the difference image may reveal the places in which the oil-and-water contact has moved indicating the areas from which oil has been pumped. If the oil-and-water contact is not changing in all expected areas of the reservoir, another well may be installed to tap into that area.
To maximize the usefulness of each seismic survey performed and minimize overall site cost, it may be desirable that every seismic survey yield new seismic data to capture further aspects of the area's subsurface. Yet, typical time-lapse surveys strive to repeat a baseline survey's source and sensor placement as closely as possible in order to compute a difference image. Thus, a new method of seismic acquisition and processing to both capture new seismic data and compute a seismic difference image may be desirable.